Hey Windy in Kansas, You've got one of the "Rare" accessory W/S Wiper Vacuum Boosters. GMC Prt. No. 2233295. Out of the 30 some, Advance Design Trucks, I've had total. Only one of them, had this Booster. It was on a '55 1st series Chevy COE, I had. I've got it out in the shop. It's exactly as you described yours. I mentioned, one of these, in the "Rarest Accessory" post, a few weeks ago.
